This invention relates to a feeder tube, for bulk products, which comprises a cylindrical drum or tube in which there are accommodated a large number of inlet openings distributed along the length of the tube, a screw conveyer disposed coaxially within the tube, a first arrangement for rotating the tube about its axis, a second arrangement for rotating the screw conveyer relative to the tube, a third arrangement for moving the feeder tube in the lateral direction, and activators for the bulk product, which activators form projections on the outer side of the tube and are disposed on the tube in association with said inlet openings.
Feeder tubes of the above stated kind are previously known and commonly used. Example of such feeder tubes are described in Swedish Patent Applications 7611862-9, 8004499-8 and in EP-751904. This kind of feeder tube, requires an extremely low amount of power input (compared to other kind of prior art feeder tubes) to the feed-out of bulk, from a buffer storage vessel, e.g. as a silo. In some known installations it has happened that jamming has occurred after shut down, which is believed to be caused by uncontrolled flow through the tube inlet openings, and flow of bulk material through the tube and thereby causing jamming of the screw conveyer.
Another disadvantage with the known kind of design is that the maximum lateral velocity of tube, within the pile of bulk products in the silo, is rather limited. A velocity exceeding 30 mm/min is mostly not feasibly, because the bulk material then acts as a barrier making it impossible to move faster even if the lateral force is drastically increased. It is evident that if the limit velocity is reached and the lateral force is increased further, this could result in undesirably high stress within some parts of the feeder tube, e.g. leading to premature wear and failure.
It has been believed that this limited lateral velocity is a kind of implicit feature of the design related to the extremely low power consumption. Accordingly it has been believed that if the velocity would be increased this could only be achieved at the price of drastically increased energy consumption. Indeed tests have been performed which imply that changes such as increasing the rotational speed of the tube and/or increasing the width of the slots have not improved the performance of the feeder tube without drastic negative consequences.
Now it has been found that considerable improvements of the performance of a feeder tube of this specific kind may be achieved by providing a feeder tube characterized in that in each cross section along at least 40% of the length of the tube, preferably at least 70%, more preferred at least 90%, there are disposed at least two inlet openings. Thanks to this arrangements the feeder tube may be moved through the bulk product at considerably higher speed than is known from prior art. Furthermore this also leads to minimised risk of jamming after shut down.
According to further aspects of the invention;
there are disposed at least three inlet openings in each of said cross sections,
said tube feeder is designed for feeding bulk products of wood chips, and/or fibrous material and/or powder, said bulk product having an average size with a maximum extension/width (X) and wherein the width (d) of each slot is 1,5-30 xc3x97, preferably 3-10 xc3x97 and more preferred 4-8 xc3x97,
in each of said cross section said openings are disposed along the circumference of the tube such that the bending resistance of said tube in said cross section is the same irrespective of which diametrical centreline (y) is chosen within said cross section, i.e. the number openings being 3 or 4 or any multiple of 3 or 4,
the diameter of said tube is between 200-1500 mm, more preferred 500-1000 mm and most preferred 600-900 mm,
said activators are detachably arranged on said tube,
the total length of said tube is between 3-30 m, more preferred 4-25 m and most preferred 10-18 m,
said first arrangement for rotating the tube about its axis provides for a rotational speed of the tube of 0-20 rpm, preferably 0,5-15 rpm and more preferred 1-8 rpm,
in a portion comprising at least 30% of the length of the tube the total sum of the width (xcexa3d) of all openings in each cross-section is between 150-700 mm, preferably at least 250 mm, more preferred at least 350 mm,
a detachably arranged cover device is positioned to cover a part of at least one of said openings such that the width (d) of said opening may be adjusted,
at least one of said activators, preferably the majority of said activators, comprises at least one discrete protruding element, preferably a number of discrete protruding elements acting as an agitator.
said tube is arranged with a number of discrete protruding elements which are positioned at a distance in relation to said inlet openings.
said activator comprises plate like structure that extends across the opening at a distance from each lengthwise extending edge of the opening in a direction substantially parallel to the extension of the tube, wherein the width (b) of said plate like structure is substantially smaller than slot width (d) of said opening, such that preferably 10 mm less than 2b less than d.